NASA Technical Reports Server (NTRS) 19910023313: Launch pad lightning protection effectiveness

N91-32627

LAUNCH PAD LIGHTNING PROTECTION EFFECTIVENESS

James R. Stahmann
Boeing Aerospace Operations
Kennedy Space Center, FL 32899

ABSTRACT

Using the striking distance theory that lightning leaders will strike the nearest grounded point
on their last jump to earth corresponding to the striking distance, the probability of striking a
point on a structure in the presence of other points can be estimated. The lightning strokes are
divided into deciles having an average peak current and striking distance. The striking
distances are used as radii from the points to generate windows of approach through which the
leader must pass to reach a designated point. The projections of the windows on a horizontal
plane as they are rotated through all possible angles of approach define an area that can be
multiplied by the decile stroke density to arrive at the probability of strokes with the window
average striking distance. The sum of all decile probabilities gives the cumulative probability
for all strokes.

The techniques can be applied to Kennedy Space Center (KSC) launch pad structures to
estimate the lightning protection effectiveness for the crane, gaseous oxygen (GOX) vent arm,
and other points. Streamers from sharp points on the structure provide protection for surfaces
having large radii of curvature. The effects of nearby structures can also be estimated.

INTRODUCTION

The launch pads at KSC are protected by a 70-foot insulating fiber glass mast 5 feet in
diameter located on the Fixed Service Structure with a lightning rod at the top of the mast.

The rod is grounded 1,000 feet north and south of the tower by a 1/2-inch stainless steel cable
called the catenary wire. The lightning protection system on Launch Pad 39A has been struck
by lightning an average of three times per year since 1979. Probability calculations predicted
about two strokes per year. The difference may be accounted for by the action of upward-going
streamers that go out to meet the down-coming leader of the lightning stroke and meet it about
100 feet above the lightning rod. This effect increases the effective height of the mast and
enhances its ability to attract strokes, especially the larger strokes. The probability of hitting
the mast, without taking streamers into account, was calculated in a previous paper [1]. The
probability of hitting selected points on the structure or the vehicle will now be considered
using a similar technique to estimate the probability of hitting the selected point in the
presence of other attracting points on the protection system, the structure, or the vehicle. This
information can be used as a factor in a management decision, such as when to start fueling in
relation to weather conditions.

Ignoring streamering effects and using the simple principle that lightning (in its last jump to
earth at its striking distance) will hit the closest point on the mast, structure, vehicle, or
ground, "windows" of approach through which the lightning must travel to reach the closest
point are created by this assumption. The windows are largest in the most vulnerable direction
of approach and then narrow and close as azimuth and stroke magnitude are changed. By
calculating the projected area of these windows on the ground and multiplying by the stroke
density for each stroke size increment (each direction increment) and summing all the incre-
ments, a cumulative probability for all strokes hitting a selected point can be determined.

Launch Pad 39B at KSC is shown is figure 1.

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Figure 1. Shuttle Vehicle on Launch Pad 39B With a Water Tower on th« Right

CRANE WINDOWS OP APPROACH

To illustrate the probability calculation process, consider the calculation of the probability of
hitting the tip of the crane boom, which is about 32.3 m (106 ft) from the lightning mast
centerline and 30.5 m (100 ft) below the mast lightning rod tip, as shown in figure 2. Arcs of
various striking distances from these two points intersect on the perpendicular bisector of the
line joining the two points. The bisector is the upper boundary of "crane windows," which are
sectors of approach where the lightning leaden are closer to the tip of the crane boom than to
the mast lightning rod or the ground when the leader makes its last jump to earth. The lower
boundary of the crane windows is a parabola with focus at the tip of the boom tip and the earth
as directrix. This boundary is the locus of pointB equidistant from the boom and the earth.
Below the parabola, all the strokes hit the earth and above it all hit the lightning mast. The
crane windows close at the striking distance where the bisector intersects the parabola. For a
mast height of 122 m (400 ft) and a boom height of 91.5 m (300 ft), this occurs at a striking
distance of 390 m (1280 ft). The windows are also the locus of the center of a "rolling ball"
(having the striking distance as radius) as it rolls over the tips of the boom and mast rod.

The technique used previously by Stahmann [1] for estimating the stroke probability divided
strokes into deciles (see table I) having average peak currents and striking distances. The
attractive radius, R, was calculated from the striking distance S d , and the height of the mast
lightning rod, H, using the relationship:

R - yj2S d H-H 2

all

( 1 )

The attractive area was calculated and then multiplied by a relatively high stroke density of 20
strokes/km 2 /yr or 2x1(1* strokes/m 2 /yr/decile to obtain the stroke probability. The lightning rod
is exposed to all strokes from all directions all year. However, to hit points below the rod (such
as the crane tip), the lightning leader must enter through the associated window to hit the
point. The windows change with direction of approach. The windows for the direction the
crane boom is pointing is shown in figure 2. In other directions, up to 90 degrees from the
crane pointing direction, the crane and mast rod tips are no longer coplanar and the distance
difference to the leader is reduced, becoming zero at 90 degrees. Therefore, in other directions,
the crane windows become smaller and close at lower striking distances. To calculate the
probability of striking the crane (see table II), the areas of the horizontal projections of the
crane windows for the various deciles were calculated as the projections were rotated ±90
degrees from the pointing direction, assuming that the projection did not change. However,
more detailed calculations indicated that this area is actually about one-half of that calculated
because the windows and projections change as the direction of approach is rotated. The
geometry of the window projected area calculations is discussed in the next section. The largest
stroke deciles are listed first in tables II and III.

GOX VENT ARM AND SOLID ROCKET BOOSTER (SRB) WINDOWS OF APPROACH

Three possible strike points are shown in figure 3, the mast rod, the tip of a rod on the GOX
vent arm, and the SRB nose below the vent arm. The geometry for calculating the probability
of hitting the vent arm is shown in figure 4 where a fourth point is added, the tip of a lightning
rod on the top of the nearby 87.2 m (286 ft) water tower 160 m (525 ft) from the lightning mast
centerline in an easterly direction, 38 degrees from the north. Figure 3 shows the east-west
plane. Lightning leaders must approach from the east to reach the vent arm or SRB tip.

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Figure 2. Crane Window* of Approach

800' VENT WINDOW

Figure 3. ET Vent Window* for Stroke* Approaching Directly From the East

Figure 4. Geometry for Calculation of the ET Vent Window Projection on a Horizontal Plane

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Table I. Probability of Striking a 122 m (400 ft) Tower

Decile

«pk

m

R

(m)

Area

(m s )

Decile

Probability

(Strokes/Year)

Cumulative

Probability

(Strokes/Year)

Years

Per

Strike

0-10

6.2 kA

46

46

6,648

0.01330

0.01330

75.2

10-20

12.9 kA

90

90

25,447

0.05089

0.06419

15.6

20-30

17.6 kA

115

115

41,548

0.08310

0.14729

6.8

30-40

22.7 kA

137

136

58,107

0.11621

0.2635

3.8

40-50

28.4 kA

161

156

76,454

0.15291

0.4164

2.4

50-60

35.2 kA

186

174

95,115

0.19023

0.6066

1.65

60-70

44.5 kA

217

195

119,459

0.23892

0.8455

1.18

70-80

57.0 kA

258

219

150,674

0.30135

1.1458 !

0.872

80-90

77.0 kA

318

250

196,350

0.39270

1.5395

0.65

90-100

112.0 kA

380

279

244,545

0.48909

2.0286

0.49

Table II. Probability of Striking the Crane

s<

(ft)

c

(ft)

a

(ft)

r i

(ft)

HI

Area

(m 2 )

Decile

Probability

(Strokes/Year)

Cumulative

Probability

(Strokes/Year)

1246

1244

853

800

811

2,587

0.005174

0.005174

1043

1040

714

661

732

14,518

0.029036

0.03421

846

843

578

525

646

20,726

0.041452

0.07566

712

708

486

433

581

21,899

0.043798

0.11946 |

610

606

415

362

525

21,129

0.042258

0.16172 1

528

523

359

306

476

19,474

0.038948

0.20067 |

449

443

304

251

424

17,015

0.034030

0.23470

377

370

254

201

369

14,010

0.028020

0.26272

295

286

196

143

295*

9,721

0.019442

0.28216

151

132

91

37.7

151*

3,122

0.006244

0.28840**

* When S d < H, r 2 = S d
** Once every 3.46 years

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Table III. Probability of Striking the GOX Vent Arm Lightning Rod

E9

a

(ft)

r i

(ft)

■

Area

(m 1 )

Decile

Probability

(Strokes/Year)

Cumulative

Probability

(Strokes/Year)

1043

772

**

717

711

Closed

—

—

846

625

**

571

629

10,181

0.020362

0.020362

712

525

**

471

566

14,424

0.028848

0.049210

610

449

**

394

513

15,716

0.031432

0.080642

528

387

**

333

466

15,540

0.031080

0.111722

449

328

**

273

416

14,362

0.028724

0.140446

377

243

**

219

364

12,374

0.024748

0.165194

295

210

344

156

295

8,691

0.017382

0.182576

151

95

204

40

150

3,036

0.006072

0.188648*

* Once every 5.3 years
** Ground stroke formula applies:

r 2 - ^SjH-H 2

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Windows are created for each of the three points. The SRB window starts to close at 74.7 m
(245 ft) and closes at 164.9 m (541 ft) in this plane. The external tank (ET) vent windows close
at about 313.7 m (1029 ft). The dimensions used are H=85.4 m (280 ft), hj=36.6 m (120 ft),
h 2 =10.7 m (35 ft), s s =33.1 m (108.5 ft), and s,=2.7 m (9 ft). The formulas used for calculating
the GOX vent arm window horizontal projected area are:

c -

(2)

a - c cos 6,

(3)

d - p<?-9 2

(4)

b - d cos 0 2 + (s 2 + s 2 ) / 2

(5)

r 1 - a - sj2

(6)

r 2 - + b - a

(7)

where r, and r 2 are the attractive radii of the upper and lower window
The maximum attraction area for each decile produced by rotating the
easterly directions ±90 degrees from directly east is:

Area(m 2 ) < 71/2 /3 .28 2

boundaries, respectively,
windows through all

(8)

where r, and r 2 are in feet. As previously mentioned, this estimate of the area is a maximum
since the windows close at shorter striking distances in other directions of approach other than
directly east. The closure formulas are:

For the GOX vent arm, closure occurs at:

S d - H + hj2 + e - yfc 2 + f 2 Solve for e and S d (9)

For the SRB, closure occurs at:

S d - H - h 2 / 2 + Jc - ^d 2 + g 2 Solve for k and S d (10)

Of particular interest is the water tower shown in figure 4. This tower appears at first glance
to offer little protection to the GOX vent arm. Actually, since the lightning leader must come in
through the vent windows in order to reach the arm, it must pass within 85.4 m (280 ft) of the
tower when approaching from a northeast direction (38 degrees). The water tower will then
capture all strokes with striking distances greater than 85.4 m (280 ft) (80 percent of all
strokes). From the south, the large rotating service structure (RSS) helps protect the ET so

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that the probability of hitting the ET is reduced in this direction. Table III gives a maximum
cumulative probability of hitting the GOX vent arm with lightning rod once every 5.3 years.
Considering window closure and other factors, the maximum probability is closer to once in 10
years.

The above assumes continuous exposure for a full year. Since the vehicle is not present at all
times and the GOX vent arm is retracted most of the time, the probability must be multiplied
by the exposure percentage per year. For example, if the exposure time is 10 percent per year,
the probability is reduced to once in 100 years or less if the exposure is less.

STREAMERING EFFECTS

An important factor in protecting the pad structures is the formation of streamers from the
sharp points on the structures. The most prominent sharp point is the tip of the rod on the
lightning mast. As shown in figure 5, the streamers go out to greet the down-coming leaders
and meet in the characteristic "knee" shown. Figure 6 shows that, while the ET ogive is
protected since it is within the NFPA 78 zone of protection based on a 30 m (100 ft) striking
distance, the ET ogive is further protected by streamers from the GOX vent arm and the tip of
the SRB. These streamers form earlier and move farther than any streamers that may form on
the ogive.

CONCLUDING DISCUSSION

The probability of lightning striking points on a structure beneath the highest point can be
estimated. Often the protected points can be approached only from a limited range of directions
and along paths that have a large horizontal component. In a particular case, such as hitting a
vehicle in the presence of a nearby structure, the structure may be modified to better protect
the vehicle by improving the protective geometry and by encouraging the formation of protective
streamers that can intercept the stroke leader using diverters in the high field regions.

REFERENCES

1. J. R. Stahmann, "Inside the Cone of Protection," International Aerospace and Ground
Conference on Lightning and Static Electricity, Fort Worth, Texas, June 21 to 23, 1983,
pp. 27-1 to 27-7.

2. R. H. Golde, "Lightning Conductor," Lightning, Volume 2, Chapter 17, Academic Press,
1977, pp. 545 to 576.

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ORiO’NX PAGE

BLACK AND WHITE PnOTOGRAPH

Figure 5. Two Strokes to the Pad Lightning Protection System Showing "Knee” Where
an Upward-Going Streamer Meets a Down-Coming Leader

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